The present invention relates to an acoustic transducer of the type comprising an electret assembly including a diaphragm mounted adjacent an electret film and a backplate. The electret assembly is mounted within a case to form acoustic chambers on opposite sides of the diaphragm. The case includes a conduit for permitting the external acoustic signal to enter into one of the acoustic chambers to enable the diaphragm to respond thereto. The backplate is perforated to permit the air pulsations created by the vibrations of the diaphragm to pass to the other acoustic chamber. And, the electret film is connected to suitable electronic circuitry to thereby permit the electroacoustical interaction of the diaphragm and electret film to provide an electrical signal representative of the acoustic signal.
As is known, the converse operation may be provided by the transducer in that an electrical signal may be applied to the electret to cause the diaphragm to vibrate and thereby develop an acoustic signal which can be coupled out of the first acoustic chamber.
A principal object of the present invention is to provide an improved construction for an acoustic transducer of the foregoing type. In the inventive transducer, the diaphragm, electret and backplate assembly are conveniently mounted on support posts formed in the case; this also forms an acoustic chamber into which external sound is admitted. The diaphragm includes flanges extending transversely from the plane of the diaphragm to permit secure adhesion thereof to the walls of the case.
The electret comprises a dielectric film deposited on a perforated backplate and the backplate includes protrusions which selectively space the dielectric film from a diaphragm. The foregoing structure permits the diaphragm, electret and backplate assembly to be mounted in the case in an acoustically efficient manner within a precise tolerance.
An electronic signal amplification circuit formed on a semiconductor die is mounted on a ceramic plate which is positioned within the case and is in part supported by a post attached to the backplate. The electronic circuit includes a pair of FET's (field effect transistors) and means for limiting current surges through the FET's. An electrical terminal comprising relatively rigid electrical leads connecting to the electronic circuit extend outwardly from the ceramic plate and through acoustically sealed apertures in the case to be connected to the associated external electronic circuitry.
The foregoing and other features, objects and advantages of the present invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings wherein: